the install process on your new powerbook's thru setup assistant (can be found in your utilisties folder) should request if you would like to recover data from previous computer you need a firewire 6pin cable to connect both in order to do so. another alternative is to start your imac in target mode using cmd+t on startup and your powerbook will mount the imac as if it was an external HD.
Now to be more specific you should let us know the kind of OS installed on both iMac and Pbook. I also encourage you to search the forum as far as target mode is concerned and/or transfering data between 2 macs